1. Field of the Invention
The present invention relates to apparatus for forming an image (on a recording medium or for viewing) from an electrical signal and more particularly to improvements in electronic imaging apparatus and method of the kind using light valves.
2. Brief Description of the Prior Art
U.S. Pat. No. 4,229,095 discloses various embodiments of electronic color-imaging apparatus that utilize arrays of separately-addressable, pixel (picture element) sized, electro-optical means to effect multicolor exposure of panchromatic imaging media. One preferred kind of electro-optical means disclosed in that patent is a light valve comprising a panel of ferro-electric ceramic material, such as lanthanum doped lead zirconate titanate (PLZT) sandwiched between crossed polarizers and activated to operate in a quadratic Kerr cell mode. Thus an array of such light valves comprises a panel of PLZT material with a plurality of electrodes formed on one major surface in a manner facilitating the selective application of discrete electrical fields across (in a direction perpendicular to the direction of viewing) discrete surface areas of the plate. Upon application of such fields, the PLZT material becomes birefringent and rotates the direction of polarization of incident light by an extent dependent on the field magnitude. This results in the transmission of light through the PLZT panel and polarizers varying as a function of the electric fields. A color image is formed electronically by selectively opening and closing of such light valves in synchronization with the energization of red, green and blue exposing sources and according to the red, green and blue color information for the pixels of that image. One preferred embodiment disclosed in the above-mentioned patent comprises a linear array of separately-addressable, light valve elements that are disposed in spaced transverse relation to the path along which a photosensitive recording medium is fed. In this embodiment the pixels of the linear array are addressed concurrently, a line at a time, with image information and the movement of the recording medium along its feed path is synchronized with successive actuations of the linear array.
Light valve arrays of the type disclosed in the aforementioned patent use relatively high voltages to drive the light valves. This can present a difficulty where it is advantageous to use integrated circuits to address the light valves. Also, high voltages can, in some instances, cause sparking and breakdown of the field of the electrodes. In high resolution imaging applications it becomes desirable to have light valve arrays with a pixel density, i.e. wherein there are a large number of discretely-addressable light valve elements per unit of distance (e.g. 100-200 per inch). In operating in such high pixel density modes, it has been noted that a problem is presented in that the addressing electric field across one pixel of the light valve array can, in certain instances, affect its adjacent pixels.
More specifically, if the image signal addressed to adjacent pixels dictates that a first be "on" (i.e. modulating light) and the second be "off" (i.e. not modulating light), non-fidelity in image reproduction will occur if the discrete electric field is applied to one of those pixels to effect its proper addressed state, also affects the other pixel, which would be addressed to have no modulating field thereacross. Various techniques for providing physical barriers to electrically isolate pixels from the exterior fields of their neighbors are possible; however, these suffer the disadvantage of decreasing the light transmitting efficiency of the array or being difficult to fabricate. The straightforward approaches for minimizing cross-talk between pixels of the array are therefore contrary to the purpose of achieving inexpensive, high-resolution arrays with good light transmitting effeciency.